Hydroxy-aluminum polymers can be prepared by the controlled addition of an alkali metal or ammonium base, e.g., sodium hydroxide or ammonium hydroxide, to an aqueous solution of a water-soluble aluminum salt. It is believed that when the base is added to an aluminum salt solution, OH.sup.- ions link the Al.sup.3+ ions together forming stable rings composed of six aluminum atoms per unit. When the molar ratio of OH/Al is in the range of 0 to 2.1, the reaction involves the formation of single units of compositions -- [Al.sub.6 (OH).sub.2 ].sup.6+ -- or double units -- [Al.sub.10 (OH).sub.22 ].sup.8+. With ratios from 2.25 to 2.7, the additional OH.sup.- reacts with these single units and forms a continuous series of higher polymers.
The nature of hydroxy-aluminum polymers and their preparation are discussed by P. H. Hsu and T. F. Bates, in Soil Science Society of America Proceedings, 28, No. 6, 763-769 (1964), and in "Formation of X-ray Amorphous and Crystalline Aluminum Hydroxides", Mineralogical Magazine, 33, 749-768 (1964).
Hydroxy-aluminum polymers are used for soil consolidation in secondary oil recovery, as disclosed, for example, in U.S. Pat. No. 3,603,399, issued Sept. 7, 1971, to M. G. Reed.
Hydroxy-aluminum polymers are also used as additives in ammonium salt compositions to reduce caking tendency and to improve hardness, as disclosed in U.S. Ser. No. 323,571, of G. R. Hawkes et al, filed Jan. 15, 1973, now U.S. Pat. No. 3,852,055.